Studded tires or snow chains on tires were used for driving on snowy and icy roads; however, they unfortunately cause environmental problems, such as dust pollution. Studless winter tires were therefore developed as alternative tires for driving on snowy and icy roads. Since studless winter tires are for use on snowy roads with rougher surfaces than normal roads, the materials and structure thereof are specially designed. For example, a rubber composition that contains a diene rubber having excellent low-temperature properties, and a rubber composition that contains a large amount of a softener to enhance the softening effect have been developed (for example, Patent Literature 1).
Some approaches have been tested to improve performance on snow and ice of studless winter tires, for example, by increasing the amount of polybutadiene rubber in a rubber composition. However, the increase in the amount of polybutadiene rubber is limited because too much an amount of polybutadiene rubber leads to excessively high mobility in the rubber, thereby causing blooming of various agents. Also, when a larger amount of polybutadiene rubber is incorporated, the proportion of natural rubber in the rubber composition decreases along with the increase in the amount of polybutadiene rubber. Consequently, the resulting rubber unfortunately has insufficient strength and poor abrasion resistance.
Another known approach to improve performance on snow and ice of studless winter tires is to incorporate glass fibers as a scratching material into a rubber composition. Unfortunately, this method may also lead to a reduction in abrasion resistance, and is therefore considered to be insufficient for improving performance on snow and ice of studless winter tires.